Here's a topic that has been on my mind of late. What is the most efficient way to drive? Here are some questions that go towards answering that question. Beware knee-jerk "obvious" answers, because I don't think the actual answers are that obvious.

* Manual or automatic?
* If manual, optimum gear to be in? Also, how to put the least strain on the clutch?
* Optimum driving speed?
* Cruise control, yes or no?

Now that these questions are presented, let me go on a bit about why the answers aren't so obvious.

* Manual or automatic:
According to my owner's manual, the transmission fluid needs to be replaced more frequently on my stick-shift than it would on an automatic. Also, I have a sneaking suspicion that manuals may need more frequent repairs. I'm guessing most of these repairs would be clutch related. I have no data to support this theory.
* Optimum gear:
Should I change gears with the engine revs as low as possible without stalling the engine? If I change gears with the RPMs very low, the enging "lugs". Does engine "lugging" wear out the engine (or other component) faster? Also, when driving in very low RPMs, I suspect the engine management dumps more fuel into each cylinder, but maybe I'm way off base on that theory.
* Changing gears with minimal strain:
I have a theory that if I release the clutch too quickly, it is putting high forces on the system. In effect, I am forcing the engine and drivetrain together more quickly. However, if I release the clutch very slowly, there is an increased period of time where the clutch "slips", allowing the engine speed to match the drivetrain. Does this wear out the clutch faster? I have read a little bit about matching engine speeds to drivetrain speeds, but am still finding this difficult to do (double declutch?).
* Optimum driving speed:
Is driving on the highest gear at the lowest speed possible the most efficient? This gets into the engine "lugging" question presented previously. Also, the slower you go, the more time you spend on the road (which means more time burning fuel). The faster you go, the more energy you waste on wind resistance.
* Cruise control:
For most people, using cruise control usually increases their fuel efficiency. However, Shell recently sponsored a husband and wife team to drive around the world on 24 tanks of gas (or something like that). This team didn't use cruise control because they maintained they can do better than just running on cruise control. If that's true, how do you drive more efficiently than a car using cruise?

There are some obvious ways to drive more efficiently:
* Right-size your car: An SUV for a two-person household is almost assuredly not efficient. However, a minivan for a senior couple may be more efficient than a small car, if they spend a lot of time carting their senior friends or grandkids around. This saves on the number of times you might have to take two cars.
* No jack-rabbit starts and coast well before stops.
* Maintaining tire pressure. Although the optimum tire pressure for fuel efficiency is not obvious, just don't run it too low.

Any additional data is appreciated.

squished

Some of your questions are answered here:

http://www.fueleconomy.gov/feg/driveHabits.shtml

And if you scroll down to the bottom of that page you'll see links to the sources they used for these conclusions, although I wish they had provided full references. I've seen a bunch of studies on these topics and will see what else I can dig up.

Looking for "optimum" efficiency is more of a thought experiment than a practicality for most of us, though. If you follow the simple guidelines on the fueleconomy site that'll get you the bulk of the gains; anything else is icing on the cake and probably not worth the effort.

Here’s another idea, use EZ-Pass (for those of you not on the east coast, it is an electronic toll paying sensor that bills your credit card). I suspect you improve fuel economy because you need only slow to 15 MPH to pay your toll. I don’t know how much fuel you'd actually use to go from 0-15 MPH, but I imagine it adds up if you drive toll roads and bridges often.

But repairs on an automatic transmission are *way* more expensive than replacing the clutch.



You probably can't beat cruise control on flat stretches. However, if you're in hilly terrain, trying to maintain an equally high rate of speed when going up major inclines can burn a lot more gas than reducing speed a bit (you're engine won't have to race as much).



Amen, brother. Also: don't haul around unnecessary (heavy) items, and lose weight.

brad,

I've heard the "fuel savings tips" quoted plenty of times. This only deals with fuel savings. I'm only convinced by data and at the moment I still have open questions regarding other factors in efficiency other than just fuel use. At this time, I disagree with your perspective that "anything else is icing on the cake and probably not worth the effort." I maintain the perspective that the government issues general guidelines that are useful for the vast majority of the population. I am a bit of an elitist and like to run more efficiently than 90% of the population, if possible.

respectfully,
squished

I drive a standard (manual) transmission car (Miata) and have noticed an uptick in my mileage based on two factors.
- One, I no longer drive in rush hour traffic, or at least not at the height of rush hour. This is not possible for all drivers, but idling bumper-to-bumper is not the way to save gas. I'm fortunate that my boss allows me to leave the office before traffic gets really bad (assuming of course I've come in on time and gotten my chores done).
- Two, and more importantly, I drive slower. I am able to set the cruise control for some of my drive, and I do so at just one-tick (2.5 mph) over the speed limit. If I drove the standard 10 miles over (which is the MINIMUM for most people here in Houston) I would only shave 1 minute off of my overall commute. But that's a theoretical 1 minute, because my driveway does not open directly onto the freeway. There are lights, stopsigns, and other cars with which to contend. Sacrificing the theoretical minute has allowed my average gas mileage to go from 23mpg to 28mpg, a 20% improvement. Yes, I'm often the slowest car on the road (even going over the speed limit), but I don't feel too bad about that as I watch gas prices climb. It is no longer a threat to my masculinity to allow another car on the road to pass me.

As for some of your other points:
- Wind resistance increases with the square of speed. Therefore, the slower you can safely drive (eg. just over the limit here), the less resistance your car must overcome and more efficient it will be.
- With few exceptions, the slower an engine turns, the less gas it burns. Therefore, the lowest gear in which you can operate your car will be the one that is most fuel efficient. The exceptions are "lugging" the engine (which is when you stomp on the gas in a low gear and the engine chugs for a second or two before revving-up) and automatic transmissions (which is only a partial-exception because you can't always control what gear the tranny will pick). Simply cruising in a high gear doesn't necessarily dump more gas into the engine, but lugging it can. That extra fuel doesn't burn completely and vaporizes out the tailpipe (along with your gas mileage).
- Cruise control is most efficient in areas of gentle grades where it smoothly applies power. In hilly terrain, the cruise control may force an automatic transmission to downshift as it climbs a steep grade.

"But repairs on an automatic transmission are *way* more expensive than replacing the clutch."

Any experience with repairs on either? How much did it cost? I've never had to have a automatic transmission repaired, nor have I ever had to have a clutch replaced.

squished

I've had a new clutch in the past and it cost a few hundred bucks. Never had to replace an AT before, but we had a Dodge Grand Caravan awhile back that had transmission trouble (under warranty, so not sure of the cost). This was the one time that we've bought an extended warranty, and were glad that we did. We sold it just before it came out of warranty in part because the transmission had given consistent trouble, and we had a sinking feeling that it was about to go entirely. A replacement would've be in the multiple thousand dollar range, but I'm not sure exactly how much. On our Honda Odyssey new transmission run about $5k.

The thing that you have to factor in is that your clutch *will* wear out. Probably more than once if you keep your car for a long time. Fortunately, it's relatively cheap. Hopefully you'll never have trouble with your AT, but if you do, the replacement cost is very large.

Ditto nickel (as is my usual reply). I agree that AT repair, on average, is more expensive than MT repair. But that's only once you actually need the repair. Clutch jobs, on average, are more frequently necessary than full-blown AT repair.

On the other hand, my Miata is almost 8.5 years old and still on the original clutch (only 67k miles). I attribute that as much to my driving style as to the flat terrain here in Houston (less slippage on hilly starts) and the fact that Mazda makes a darn good car. By contrast, I had an RX-7 15 years ago that REGULARLY needed a clutch job (due to both my driving and living in hilly Austin). My sister once burned up that clutch in a week.

My first car was a hand-me-down Pontiac Grand Prix that I later heard Consumer Reports rated as the worst (automatic) transmission ever put into a car. It needed a new transmission at least once in the 3 years I had that beast. As usual, your mileage may vary.

Yeah, your clutch wears out as a normal function of doing its job. Your AT *shouldn't* fail, but it's a beast when it does. So you really have to weigh the alternate scenarios by the likelihood that they'll happen.

If you move to Iowa, however, you can probably keep your clutch in good shape for a long, long time. Both because it's totally flat, and because there's nowhere worth driving anyway...

Interesting thoughts all around. I never realized that the clutch is a part designed to wear out, like brake pads. So the point is to minimize clutch slippage by releasing the clutch as quickly as possible without making the car jerk or hop, right? And slower starts are easier on the clutch than jack-rabbit starts. Makes sense.

http://www.aa1car.com/library/2004/bf40459.htm

It doesn't have to wear out very often, though. If I remember correctly I replaced the clutch only once on my 1990 Honda Civic wagon, and that was after I'd driven it more than 200,000 miles (during 10 years when I lived four miles up a steep dirt road in Vermont).

For maintenance on auto transmissions, I have owned two relatively older Honda Accords - my previous was an '90, and my current is an '88. The '90's clutches (the auto trannys have clutches within the tranny box itself) were starting to slip, and so what I did was do a full fluid flush every year. That cost me about $150. When I sold the car, it was about time to replace that thing, and I had found a replacement really cheaply at $700. And part of that is that I work in the auto industry, and so automatically get discounts...... I do remember that the replacement with the Honda rebuilt tranny would have cost somewhere around $3700. And the car had just under 200K on it when I sold it.

So it all comes down to the type of car you drive....how reliable is that tranny? The 90 Accord coupe was going at about 200K, and my current '88 Accord has issues, but these issues have been known since inception I am almost sure it won't be replaced while I still own it.

It also depends on what the clutch replacements cost on your manual....How often has the car needed it up until now, and what was the average cost over time? That would tell at least which transmission is more friendly, repair/cost-wise.

As for a good read on mileage, and getting the best ever possible, I would suggest reading that article that JD quoted a while back. I don't remember what the article was, but it was funny! And just a bit scary what this guy did for his mileage.....But it did come up with some good ways of heightening gas mileage - like don't idle at a light for more than 30 seconds. I believe it is the Norwegians who actually just shut their cars off when idling. Interesting idea...wish it were possible in self-centered/must-move-faster-than-anyone-else-because-I'm-more-important San Diego... (I think it'll be the drivers who get me to move out of here! Oh how I loved Portland and its conscientious citizens...)

~Jamie

Anybody have a link to this article?

Of course, this topic requires me to link the article about the guy JD posted in March.
http://www.getrichslowly.org/blog/2007/ ... the-world/

And then there's an article from HowStuffWorks:
http://auto.howstuffworks.com/how-to-dr ... ically.htm